Liquid level or film detection devices are found to have broad utility in industrial as well as domestic application. In one aspect, the devices exhibit important utility in monitoring predefined areas for the detection of excursions in liquid conveying systems. As an example, the rigid humidity controls necessarily present in many industrial environments, e.g. computer rooms and the like, require the presence of water conveying conduits which conventionally are positioned in a subfloor enclosure which also serves to retain an extensive concentration of electrical conduiting and the like. Ruptures of pressurized water lines in such environments and others can result in extensive damage to equipment, including important data retention componemts, in the absence of a prompt and adequate warning of excursions at the incipient stage of any given failure. In domestic applications, a need has been recognized for an inexpensive, very easily installed device which is operative to detect and provide a form of alarm upon the incidence of sudden or progressive failure in water retaining and/or conveying implements such as boilers, water heaters, sump pumps, clogged air conditioning condensate drains, dishwashers, clothes washers, as well as the incidence of flood water, foundation leaks or sewer back-up conditions. Similarly, the devices find utility in marine use, with respect to boat holds, engine compartment flooding, bilge pump failures and the like. Consumer market as well as industrial market acceptance of devices for providing incipient state warning of water system failure requires that such devices exhibit high standards of reliability coupled with an extended term performance capability. In the latter regard, owners in the consumer field cannot be expected to frequently inspect and test all components within a water or moisture excursion monitoring system. Only the most basic and simple inspection routines can be expected of the domestic user. Accordingly, incipient failure warning devices should exhibit designs permitting highly convenient access to the use for such routine maintenance procedures as battery testing and/or replacement as well as testing of any audible or perceptible alarm components. Usually, however, the locations or sites at which testing devices are located necessarily are remote from the user, leading to the neglect of maintenance procedures. Techniques wherein more or less lengthy electrical leads are coupled to sensors from a remotely housed power supply and logic circuit have exhibited reliability defects stemming from R.F. interference and the like associated with the use of such long cable type interconnections.
Another aspect detracting from the acceptability of typical detection and warning devices stems from the galvanic action induced corrosion of probes which are energized from direct current type power supplies. This electrolytic-type action greatly lowers the lifespan of such probes to the extent that the operational reliability, even of monitoring and alarm devices which normally are not within a liquid environment, is considerably diminished. Further, the design criteria for such devices dictates a simplicity such that unit costs can be achieved in their manufacture permitting wide consumer market acceptance. Where reliability factors additionally must be built into the systems, such cost considerations become difficult to accommodate.